Playing with Isomerism: Cocrystallization of Isomeric N

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Playing with Isomerism: Cocrystallization of Isomeric NSalicylideneaminopyridines with Perfluorinated Compounds as Halogen Bond Donors and Its Impact on Photochromism Andrea Carletta, Marija Zbacnik, Mégane Van Gysel, Matea Vitkovi#, Nikolay Tumanov, Vladimir Stilinovi#, Johan Wouters, and Dominik Cin#i# Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01064 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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Crystal Growth & Design

Figure 1. Tautomers of N-salicylideneaminopyridine derivatives (labeling scheme of the heteroatoms is reported for the enol form). 29x10mm (600 x 600 DPI)

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Chromophores (ovan3amp and ovan4amp) and co-formers used in the present work 92x106mm (600 x 600 DPI)


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Figure 3. Full interaction maps calculated on (a) ovan3amp and (b) ovan4amp derivatives. Interaction maps are calculated for carbonyl (red), uncharged NH (blue), C-I (purple) and C-F (green) probes. 85x112mm (600 x 600 DPI)



Figure 4. Ball-and-sticks representation of the structural halogen-bonded motifs in cocrystals of ovan3amp with 12tfib (a), 13tfib (b), 14tfib (c), ofib (d) or 135tfib (e) and of ovan4amp with 12tfib (f) 14tfib (g) 135tfib (h). 197x279mm (600 x 600 DPI)


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Figure 5. Diffuse reflectance spectra of (ovan3amp)2·135tfib (top) showing thermal fading of the orangered coloration. Points at 500 nm were plotted over the time and fitted by a bi-exponential curve (middle). Repeating photochemical recovery over 5 switching cycles (bottom). 168x354mm (600 x 600 DPI)



Figure 6. Packing view along the a-axis of (a) (ovan3amp)2·14tfib (blue sticks) and (ovan3amp)2·12F8C4 (green sticks). The two solids are isomorphous. 124x192mm (600 x 600 DPI)


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Playing with Isomerism: Cocrystallization of Isomeric NSalicylideneaminopyridines

with

Perfluorinated

Compounds as Halogen Bond Donors and Its Impact on Photochromism

Andrea Carletta1, Marija Zbačnik2, Mégane Van Gysel1, Matea Vitković2, Nikolay Tumanov,1 Vladimir Stilinović2, Johan Wouters*,1, and Dominik Cinčić*,2

1

Department of Chemistry, Namur Institute of Structured Matter (NISM), University of

Namur, 61 rue de Bruxelles, B-5000 Namur, Belgium E-mail: [email protected] 2

Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, HR-

10000 Zagreb, Croatia. E-mail: [email protected]

Abstract In N-salicylideneaniline derivatives, photochromism occurs by a two-step isomerization mechanism. Photochromism has been reported to be closely related to the molecular conformation of the N-salicylideneanilines (described by the dihedral angle Φ) and to the free available space (Vfree) in the crystal. In this contribution, we focus on cocrystals of isomeric N-salicylideneaminopyridines with perfluorinated halogen bond donors as co-formers. The advantage of working with isomers is that they have a similar (if not equal) molecular volume. This aspect means that the evaluation and comparison of the free available spaces between cocrystals is not affected by the differences in the molecular volumes of their constituents. Cocrystals were synthesized by a one-pot procedure which allowed concomitant formation of covalent (imine) and non-covalent (halogen) bonds. Each cocrystal was characterized by Xray diffraction and its photochromism was assessed by UV-Vis diffuse reflectance. Our results suggest that the Φ/Vfree model, grossly oversimplifies the complexity of the dynamic phenomenon behind the photoisomerization of N-salicylideneanilines. For the above1 ACS Paragon Plus Environment


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mentioned reasons, careful examination of intermolecular interactions and crystals packing (localization of voids, evaluation of intermolecular interactions and stacking modes) should be taken into account and used to supplement the current proposed model based on Φ and Vfree.

Introduction Since its first introduction by G. Schmidt in 1971, the term ‘crystal engineering’ has broadened considerably, encompassing various aspects of supramolecular chemistry.1 The ultimate goal of crystal engineering lies in the design of solids with desired physical and chemical properties through the study of intermolecular interactions and crystal packing.2–5 Nowadays, cocrystallization (the phenomenon of aggregation of two or more chemical entities in the same crystal lattice) has become a popular tool for the improvement of physical and chemical properties of pharmaceuticals.6–8 However, applications of cocrystals go beyond pharmaceuticals. In fact, over the past few years, we have witnessed a growing interest in the development of cocrystallization procedures to generate luminescent9–11 or smart photo-,12–17 thermo-,16–22 mechano-,19,23 solvato-,24 and vapo-responsive19,25 solids but also dichroic26–28 and energetic materials.29–31 In this work, we will be focused on the crystal engineering of solid-state photochromism through cocrystallization with classical perfluorinated halogen bond (XB) donors.32 Over the past several decades halogen bonds33 have been well recognized in crystal engineering18,34,35 and are among the most interesting non-covalent interactions36, of the family of σ-hole interactions.37,38 They have been used for constructing supramolecular assemblies such as cocrystals,39–41 and functional materials.42–44 Photochromism is a light induced reversible change of color due to the transformation of a chemical species into another having different absorption spectrum.45 We will investigate the effects of cocrystallization on selected cocrystals of N-salicylideaminopyridine derivatives. These derivatives belong to a class of compounds, the imines of salicylaldehyde (N-salicylideneanilines), which can manifest photochromism in the solid state.46,47 The proposed two-step mechanism (Figure 1) consists of an excited-state proton transfer (enol to cis-keto) and a subsequent cis-trans photoisomerization (cis-keto to trans-keto).

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Figure 1. Tautomers of N-salicylideneaminopyridine derivatives (labeling scheme of the heteroatoms is reported for the enol form). Despite this well-known mechanism,45,48–50 much remains to be learned about factors that control the expression of photochromism in the solid state. The first proposed factor for the prediction of photochromism was identified in the molecular conformation of the Nsalicylideneaniline and interplanar separation between these molecules in the crystal packing.50 Jomoto et al. supported this hypothesis by a systematic investigation on salts and cocrystals of two N-salicylideneanilines.17 They concluded that when the conformation is “non-planar” – that means that the dihedral angle between the two aromatic rings is more than 30° - the N-salicylideneanilne will likely be photochromic. Instead, when the dihedral angle (Φ) is less than 20° the solid will be non-photochromic. However, several exceptions to this rule were later reported.16,51–54 Another descriptor of photochromism was proposed by Garcia et al51: the “free available space” (Vfree). The idea behind these two models is that the less compact the crystal packing, the more likely the solid will be photochromic. These models were recently reviewed by Sugijyama and Uekusa in a new series of cocrystals of Nsalicylideneaminopyridine derivatives.55 In this study, they suggest that the combined evaluation of Vfree and of Φ improves the description and the prediction of photochromism. To go deeply in this investigation, we decided to work on two isomeric N-salicylideneanilines which were cocrystalized with a series of halogen donors (three of which themselves are isomeric diiodoperfluorobenzenes). One of the advantages of working with isomers is that they can have similar (if not equal) volume requirements. This aspect means that the free available space will strictly be a function of the molecular packing and not of the differences in the molecular volumes of the constituents of the cocrystal. The two selected isomeric Nsalicylideneaminopyridines, namely ovan3amp and ovan4amp (Figure 2), are obtained from condensation reaction between o-vanillin and 3- or 4-aminopyridine, respectively. Cocrystal

formers

employed

diiodotetrafluorobenzene

(14tfib)

(Figure and

2)

are

its

meta-

halogen and

bond

donors

ortho-isomers

1,4(1,3-

diiodotetrafluorobenzene, 13tfib, and 1,2-diodotetrafluorobenzene, 12tfib). We have also included in this study the tritopic co-former 1,3,5-triiodo-2,4,6-trifluorobenzene (135tfib), and 1,4-diiodooctafluorobutane (ofib). The latter is a 180° ditopic XB donor, like 14tfib, giving the possibility to obtain two isomorphous cocrystals. We will provide a systematic structural investigation with a thorough characterization of photochromism by UV-Vis 3 ACS Paragon Plus Environment


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diffuse reflectance spectroscopy. Parameters currently proposed for the rationalization of photochromism (Φ and Vfree) were determined for each cocrystal.

Figure 2. Chromophores (ovan3amp and ovan4amp) and co-formers used in the present work

Experimental Section Cocrystal one-pot mechanosynthesis and crystallization As the mechanochemical synthesis of imines56–58 and their halogen-bonded cocrystals15,59,60 has been previously described as an efficient synthetic approach, we performed cocrystal screening by grinding.61,62 N-salicylideneaminopyridine-based cocrystals were synthesized by mechanochemistry in a one-pot single-step procedure. All three reactants (o-vanilline, an aminopyridine and a halogenated co-former) were milled in a ball mill jar. The assynthesized powders were characterized by means of powder X–ray diffraction. The experimental patterns obtained were compared with the ones calculated from the singlecrystal structures to identify the output of the synthesis. Besides the low-waste characteristics of the mechanochemical approach, the application of this one-pot single-step synthesis has two important advantages: its timesaving properties and the easiness to deal with sticky paste products. The classical synthesis of N-salicylideneanilines (condensation reaction between an aldehyde and an aniline) is usually performed on a Dean–Stark apparatus. This procedure requires relatively long reaction times (typically 8-24 hours) and proceeds at reflux temperature. The product is typically recovered by crystallization/precipitation to afford 6085% yields.52,54 These condensation reactions require less than 60 minutes when performed by mechanochemistry (at 30 Hz) in stainless-steel grinding jars and the chemical conversion is near-total (Figure S1-S2). One issue of the ball milling is that sticky products can arise during the milling procedure. The products may stick to the balls or the sides of the milling jar, reducing the energy transfer and thus leading to inefficient mixing or milling and therefore lower conversion/reaction yields. For instance, compound ovan4amp (synthetized from o-vanillin and 4-aminopyridine) appears as a viscous red oil in which the miller balls are trapped. Adding the halogenated cocrystal former to the pre-synthetized ovan4amp results in a sticky paste showing traces of a viscous red oil. Instead, we noticed that 4 ACS Paragon Plus Environment

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simultaneous addition and milling of o-vanillin, 4-aminopyridine and the halogenated coformer led to high conversion of cocrystal (Figure S1). In all cases, free-flowing powders were obtained. Detailed synthetic procedures are reported below. Powder diffractograms are reported in Figure S1-S2. Mechanosynthesis of ovan3amp·12tfib. o-Vanillin (182.0 mg, 1.20 mmol), 3-aminopyridine (112.1 mg, 1.20 mmol) and 12tfib (481.2 mg, 1.20 mmol) were placed in stainless steel grinding jars (25 mL). Mechanosynthesis was performed during 60 minutes at 30 Hz. A yellow-orange powder product was obtained. Its powder pattern matched the one simulated from single-crystal X-ray data (Figure S2). Yellow-orange plates suitable for structure determination were obtained in a 50:50 mixture of EtOAc/cyclohexane. Mechanosynthesis of ovan3amp·13tfib. o-Vanillin (364.0 mg, 2.40 mmol), 3-aminopyridine (224.2 mg, 2.40 mmol) and 13tfib (380 µl, 2.40 mmol) were placed in stainless steel grinding jars (25 mL). Mechanosynthesis was performed during 60 minutes at 30 Hz. An orange-red powder product was obtained. Its powder pattern matched the one simulated from singlecrystal X-ray data (Figure S2). Orange-red platy crystals suitable for structure determination were obtained in a 50:50 mixture of EtOAc/cyclohexane. Mechanosynthesis of (ovan3amp)2·14tfib. o-Vanillin (370.6 mg, 2.42 mmol), 3aminopyridine (227.7 mg, 2.42 mmol) and 14tfib (486.3 mg, 1.21 mmol) were placed in stainless steel grinding jars (25 mL). Mechanosynthesis was performed during 60 minutes at 30 Hz. An orange-red powder product was obtained. Its powder pattern matched the one simulated from single-crystal X-ray data (Figure S2). Yellow-orange needle-shaped crystals were obtained in a 50:50 mixture of EtOAc/cyclohexane. Crystal structure of (ovan3amp)2·14tfib is retrieved from the literature (CSD refcode: SEDFIT0115) Mechanosynthesis of (ovan3amp)2·ofib. o-Vanillin (364.0 mg, 2.40 mmol), 3-aminopyridine (224.2 mg, 2.40 mmol) and ofib (219 µl, 1.20 mmol). Mechanosynthesis was performed during 60 minutes at 30 Hz. An orange-red powder product was obtained. Its powder pattern matched the one simulated from single-crystal X-ray data (Figure S2). Yellow-orange needle-shaped crystals were obtained from chloroform. Mechanosynthesis of (ovan3amp)2·135tfib. o-Vanillin (175.7 mg, 1.15 mmol), 3aminopyridine (108.2 mg, 1.15 mmol) and 135tfib (293.1 mg, 0.58 mmol) were placed in stainless steel grinding jars (25 mL). Mechanosynthesis was performed during 60 minutes at 30 Hz. A yellow-orange powder product was obtained. Its powder pattern matched the one



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simulated from single-crystal X-ray data (Figure S2). Yellow-orange needle-shaped crystals suitable for structure determination were obtained in a 50:50 mixture of EtOAc/cyclohexane. Mechanosynthesis of (ovan4amp)2·12tfib. o-Vanillin (419.6 mg, 2.74 mmol), 4aminopyridine (257.9 mg, 2.74 mmol) and 12tfib (551.4 mg, 1.37 mmol) were placed in stainless steel grinding jars (25 mL). Mechanosynthesis was performed during 60 minutes at 30 Hz. An orange powder product was obtained. Its powder pattern matched the one simulated from single-crystal X-ray data (Figure S1). Orange prism-shaped crystals suitable for structure determination were obtained by slow evaporation from a saturated EtOAc solution. Mechanosynthesis of (ovan4amp)2·14tfib. o-Vanillin (457.7 mg, 2.99 mmol), 4aminopyridine (281.3 mg, 2.99 mmol) and 14tfib (600.9 mg, 1.50 mmol) were placed in stainless steel grinding jars (25 mL). Mechanosynthesis was performed during 60 minutes at 30 Hz. An orange powder product was obtained. Its powder pattern matched the one simulated from single-crystal X-ray data (Figure S1). Orange prism-shaped crystals suitable for structure determination were obtained by slow evaporation from a saturated EtOAc solution. Mechanosynthesis of ovan4amp·135tfib. o-Vanillin (195.6 mg, 1.28 mmol), 4aminopyridine (120.6 mg, 1.28 mmol) and 135tfib (651.1 mg, 1.28 mmol) were placed in stainless steel grinding jars (25 mL). Mechanosynthesis was performed during 60 minutes at 30 Hz. An orange powder product was obtained. Its powder pattern matched the one simulated from single-crystal X-ray data (Figure S1). Orange prism-shaped crystals suitable for structure determination were obtained by slow evaporation from a saturated EtOAc solution.

Single-crystal X-ray diffraction. Single-crystal diffraction data for ovan3amp·12tfib (293 K), (ovan3amp)2·135tfib (293 K) and (ovan3amp)2·ofib (100 K) were collected on an Oxford Diffraction Gemini Ultra R system (4-circle kappa platform, Ruby CCD detector) using Mo Kα (λ = 0.71073 Å) radiation. Diffraction measurements for ovan3amp·13tfib (298 K), ovan4amp·12tfib (293 K), (ovan4amp)2·14tfib (150 K) and ovan4amp·135tfib (298 K) were performed on an Oxford Diffraction Xcalibur Kappa CCD X-ray diffractometer using Mo Kα radiation. The structures were solved by SHELXT63 and then refined by full-matrix least square refinement of |F|2 using SHELXL-2016.64 Non-hydrogen atoms were refined anisotropically; hydrogen atoms were located from difference Fourier map. Hydrogen atoms non-involved in hydrogen bonding were refined in the riding mode with isotropic temperature 6 ACS Paragon Plus Environment

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factors fixed at 1.2Ueq of the parent atoms (1.5Ueq for methyl group). Coordinates of the hydrogen atoms implicated in hydrogen bonds were refined. The program Mercury65 was used for molecular graphics. Structure (ovan3amp)2·14tfib (115 K) was retrieved from the CCDC database (SEDFIT0115). CSD refcodes: 1547940, 1548140, 1548172, 1552262, 1582770, 1850125, 1850126.

Powder X-ray Diffraction Measurements. X-ray powder patterns (Cu Kα radiation, step size 0.017°; 45 mA, 30 kV) were collected in the 2θ range 5-40° using a Panalytical X’Pert PRO diffractometer (Bragg-Brentano geometry, X’Celerator detector). The program Mercury was used for calculation of X-ray powder patterns from single crystal structure data.

Full interaction maps. Full Interaction Maps66 (FIMs) were used to explore the hydrogen bond and halogen bond donor/acceptor areas in molecules ovan3amp and ovan4amp. Chromophore molecules used for calculation of FIMs were taken from the structures (ovan3amp)2·135tfib and ovan4amp·135tfib, respectively. FIMs are density maps indicating donor/acceptor interaction propensities of functional groups onto whole molecules taking into account steric factors. FIMs are generated from 3D Isostar scatterplots of individual pairs of functional groups. Carbonyl probe (used to find H-bond donor hotspot onto the molecule), uncharged NH probe (used to find H-bond acceptor hotspot onto the molecule), C-F probe (used to find hotspots for the interaction with fluorine atoms of the coformer) and C-I probe (used to find halogen bond acceptor hotspot onto the molecule) were used to generate the FIMs.

Calculation of Vfree and molecular volume. Molecular volumes (needed for the calculation

of

Vfree)

were

calculated

from

the

Molinspiration

free

toolkit

(www.molinspiration.com).67 These molecular volumes have been obtained by fitting sum of fragment contributions to 3D volume (fully optimized by the semiempirical AM1 method) for a training set of about twelve thousand molecules.67 Vfree was therefore calculated as reported by Garcia and co-workers.51 In cases where compared cocrystals presented the same molecular formula, the density (ρ) obtained from the single-crystal X-ray diffraction experiments is also provided.

UV-Vis diffuse reflectance. Measurements were performed on pure microcrystalline powder samples on a Varian 5E spectrophotometer equipped with a “praying mantis” diffuse reflection accessory. Measured reflectance spectra of solid samples were converted to 7 ACS Paragon Plus Environment


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absorption spectra using the Kubelka–Munk function.68 Photochromism investigations were performed by irradiating powder samples by using a MAX-303 lamp (Xenon light source 300W) at 360 nm (by use of 20 nm bandpass filters) during 15 minutes. Thermal fading was analyzed by monitoring the evolution of absorption at 500 nm in a dark environment. Spectra were measured each 4 min over 180 min, in a dark environment for (ovan3amp)2·135tfib. For the slowest thermal decaying solid, ovan3amp·12tfib, spectra were taken each 20 min for 2160 min (36 h). Spectra of (ovan3amp)2·ofib were measured each 20 min over 800 min.

Results and Discussion Structural Characterization The

chromophores

we

have

selected

belong

to

the

family

of

the

N-

salicylideneaminopyridines. These compounds offer important advantages for what concerns the construction of multicomponent crystal forms with respect to the classical Nsalicylideneaniline derivatives. In particular the N-atom of the pyridine moiety can act as a strong H- or X-bond acceptor making the formation of cocrystals easier through these interactions.15,16,55,59 Alternatively, the N-atom can be easily alkylated to form various pyridinium salts.69 The vanillidene moiety has also been shown to be involved in halogen and hydrogen bonds in which both the two O-atoms may be involved.15,16,57 In particular, the presence of the methoxy group has been shown to increase the tendency toward cocrystal formation given that it can act as an “ancillary” acceptor to generate slightly bifurcated halogen bond geometries or a symmetrical bifurcated halogen bond.60 In some derivatives this acceptor area interacts with the H–C(sp2)

iminic hydrogen of a second N-

salicylideneaminopyridine molecule. Full interaction maps calculated for carbonyl (red), uncharged NH (blue), C-F (green) and C-I (purple) probes on ovan3amp (Figure 3a) and ovan4amp (Figure 3b) provide a 3D visualization of the interaction preferences for these two molecules. Combination of these donor and acceptor properties contributes to a variety of supramolecular assemblies differing in type (chains, rings or discrete 2- or 3-component assemblies) and composition (N-salicylideneaminopyridine and the co-formers are usually assembled in a 1:1 or 2:1 stoichiometry) in which the N-salicylideneaminopyridine derivative is involved. For these reasons, N-salicylideneaminopyridines are versatile systems for the construction of an extensive library of multicomponent systems.


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Figure 3. Full interaction maps calculated on (a) ovan3amp and (b) ovan4amp derivatives. Interaction maps are calculated for carbonyl (red), uncharged NH (blue), C-I (purple) and CF (green) probes.

Compound ovan3amp formed cocrystals with each of the five selected co-formers. For all of them structure determination by single-crystal X-ray diffraction was possible. Cocrystals exhibit

a

1:1

or

(ovan3amp)2·14tfib,15

2:1

stoichiometry:

(ovan3amp)2·ofib

ovan3amp·12tfib,

and

ovan3amp·13tfib,

(ovan3amp)2·135tfib.

When

the

stoichiometry is 2:1, the supramolecular synthon found is the I···N halogen bond directed toward the pyridine moiety of the chromophore. This condition generates discrete 3membered assemblies constituted by two molecules of chromophore bridged by a co-former molecule (Figure 4c-d). When, instead, the stoichiometry is 1:1 the vanillidene moiety is also involved in the interaction with the co-former (I···O synthon). In this case, the supramolecular assemblies are rather chains or 4-membered rings (Figure 4a-b). Compound ovan4amp formed cocrystals with 3 of the 5 selected co-formers. These cocrystals are: (ovan4amp)2·12tfib, (ovan4amp)2·14tfib, ovan4amp·135tfib (Figure 4f-h). Crystallographic parameters of structure determination and refinement are provided in Table S1. Halogen bond geometries are provided in Table 1.

Figure 4. Ball-and-sticks representation of the structural halogen-bonded motifs in cocrystals of ovan3amp with 12tfib (a), 13tfib (b), 14tfib (c), ofib (d) or 135tfib (e) and of ovan4amp with 12tfib (f) 14tfib (g) 135tfib (h).

Table 1. Geometry of halogen bonds in the prepared cocrystal structures (numbering scheme is reported in Figure 1). Compound Name ovan3amp·12tfib ovan3amp·13tfib

Contact type A···I N2···I O1···I O2···I N2···I O1···I

d(A···I) / Å

A···I-C angle / °

3.014(5) 3.473(4) 3.315(4) 2.861(4) 3.066(3)

163.2(2) 152.8(1) 156.1(1) 173.2(1) 175.2(1) 9



O2···I N2···I N2···I N2···I O1···I O2···I

3.457(3) 2.855(6) 2.772(2) 2.902(4) 3.284(4) 3.154(4)

135.8(1) 168.5(2) 176.7(1) 178.3(2) 151.9(2) 157.3(2)

N2···I (second molecule)a

2.889(5)

175.1(2)

N2···I

2.838(6)

172.3(2)

(ovan4amp)2·12tfib

N2···I (second molecule)a

2.962(5)

161.5(2)

(ovan4amp)2·14tfib ovan4amp·135tfib

N2···I N2···I

2.812(2) 2.829(4)

170.3(1) 172.1(1)

(ovan3amp)2·14tfib (ovan3amp)2·ofib

(ovan3amp)2·135tfib

a

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cocrystals characterized by two crystallographically independent molecules in the asymmetric unit.

Solid-state photochromism The photochromic behavior was investigated by irradiating the microcrystalline powders at 365 nm. An N-salicylideneaniline derivative is photochromic if irradiation is followed by a change in its visible absorption spectrum. An increase of the 450-600 nm spectral region ascribed to the formation of the photoproduct, the trans-keto form, is usually observed. This change is consistent with a yellow-to-red or orange-to-red color transition. However, cases of negative photochromism (characterized by a decrease of the 450-600 absorption band) have been reported for some N-salicylideneanailine derivatives.70,71 Compound ovan3amp is non-photochromic as early reported.16 Compound ovan4amp is a viscous oil at room temperature, therefore characterization of photochromism of this chromophore is out of the scope of this work. Solid (ovan3amp)2·14tfib is reported to be non-photochromic.15 Among the cocrystals presented in this study, 3 out of 8 are photochromic and present the characteristic increase in the 450-600 spectral region (Figure 5 and Figure S3-S4). For the remaining 5 cocrystals, irradiation at 365 nm did not induce any change in the coloration (and absorption spectra) of the solid compounds. Cocrystals which showed photochromism at solid-state are: ovan3amp·12tfib, (ovan3amp)2·135tfib and (ovan3amp)2·ofib (Figure 5 and Figure S3-S4). After irradiation, the initial coloration of the solid can be thermally recovered by leaving the powder in the dark. This thermal decay (or fading) of the coloration was fitted by a biexponential equation.72,73 This is rather frequently 10 ACS Paragon Plus Environment

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encountered for N-salicycilideneanilines and it is attributed to the two two-stage decay of the trans-keto form. However, single exponential decay has been reported for some derivatives of the series.17,52 Compound ovan3amp·12tfib is characterized by a slow thermal fading of the coloration fitting a biexponential curve with τ1 = 124 ± 11 min and τ2 = 3211 ± 364 min at 298 K (Figure S4, Table 2). The compound showed to be stable during 5 photoinduced (15 min irradiation at λ = 550 nm) switching cycles. Compound (ovan3amp)2·ofib thermal fading fits a biexponential curve with τ1 = 126 ± 12 min and τ2 = 448 ± 25 min at 298 K (Figure S3, Table 2). However, tests of photochemical recovery of the coloration over 5 switching cycles revealed that the photoswitching ability of (ovan3amp)2·ofib is already affected during the 3rd cycle. Thermal fading of (ovan3amp)2·135tfib fits a biexponential curve with τ1 = 4 ± 0.1 min and τ2 = 63 ± 4 min at 298 K. Its initial coloration could also be recovered photochemically (upon excitation at λ = 550 nm) during repetitive photoinduced switching cycles (Figure 5, Table 2).

This

aspect

evidences

a

good

fatigue

resistance

and

photostability

of

(ovan3amp)2·135tfib.

Figure 5. Diffuse reflectance spectra of (ovan3amp)2·135tfib (top) showing thermal fading of the orange-red coloration. Points at 500 nm were plotted over the time and fitted by a biexponential curve (middle). Repeating photochemical recovery over 5 switching cycles (bottom).

Table 2. Time constants for the three photochromic solids at 298 K. A1 and A2 (in parentheses) are the relative weight of the preexponential factor. ovan3amp·12tfib

(ovan3amp)2·ofib


τ1 / min (A1)

124 ± 11 (0.19)

126 ± 12 (0.26)

4 ± 0.1 (0.03)

τ2 / min (A2)

3211 ± 364 (0.81)

448 ± 25 (0.74)

63 ± 4 (0.97)



Page 18 of 31

In-depth structural comparison: Predictability of photochromism based on Φ and Vfree. Here follows a more detailed structure comparison in which the commonly used descriptors of the likelihood of solid-state photochromism will be provided. Those descriptors are the dihedral angle Φ and the average available free space (Vfree). It has been proposed that when the conformation is “non-planar” – when Φ > 30° – the N-salicylideneaniline will likely be photochromic. Instead, when the dihedral angle (Φ) is less than 20°, the solid will likely be non-photochromic.17,45,50,55,74 For the Vfree, no general threshold value beyond which the solid is photochromic has so far been proposed. However, it has been suggested that the bigger Vfree, the more likely the solid will be photochromic.51,55 One of the advantages of working with structural isomers is that they can have (almost) identical molecular volumes. The three isomeric co-formers used in this work (12tfib, 13tfib and 14tfib) are all planar molecules in which the only variation is the disposition of the halo-atoms. Calculated molecular volumes for the three isomeric co-formers is about 152 Å3. Calculated molecular volume for the two chromophores ovan3amp and ovan4amp is of 208 Å3. This aspect means that the free available space will strictly be a function of the molecular packing and not of the differences in the molecular volumes of the constituents of the cocrystal. For this purpose, we also decided to give particular attention to the comparison of cocrystals having the same molecular formula (therefore same stoichiometry). The examination of these cocrystals of isomers as groups gives helpful information about the features of molecular geometry, free volume and intermolecular interactions, that differentiate these structures from each other. This information will be used to derive a relation between structure and solid-state photochromism. When comparing compounds having the same molecular formula, the experimental crystal density (ρ) calculated from the X-ray diffraction experiments is an evaluation of the compactness of the crystal packing. 1:1 cocrystals with ditopic isomeric co-formers: ovan3amp·12tfib and ovan3amp·13tfib. Molecules of ovan3amp display a planar conformation in both crystal structures. The dihedral angle, Φ, is of 13.0(6)° in ovan3amp·12tfib and of 5.1(4)° in ovan3amp·13tfib. Only ovan3amp·12tfib is photochromic. Solid ovan3amp·12tfib is an exception to the Φ rule: despite its planar conformation, the compound is photochromic. Another aspect to take into account is Vfree: ovan3amp·12tfib is the solid with the higher density (and lower Vfree, Table 3) of the two compounds. These results are both in contradiction with the current 12 ACS Paragon Plus Environment

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model proposed for the prediction of photochromism of N-salicylideneanilines based on the sole evaluation of the free available space parameter (Vfree) and of the dihedral angle Φ. Table 3. Molecular, packing parameters and experimentally determined photochromism for ovan3amp·12tfib and ovan3amp·13tfib ovan3amp·12tfib ovan3amp·13tfib

Vfree / Å3

Φ/° 13.0(6)° 5.1(4)°

136 161

ρ / (g cm-3) 2.110 2.010

photochromism Yes No

In both crystal structures, ovan3amp is bound through halogen bonds to its two sides: vanillidene and aminopyridine (I···O and I···N type, respectively). In the case of ovan3amp·12tfib, molecules are arranged following infinite chains in which the two constituents alternate. In the case of ovan3amp·13tfib, we observe a finite 4-membered ring arrangement (Figure 4). Closer analysis of the XB distances (Table 1) and angles reveals the “poor” directionality of XBs in ovan3amp·12tfib when compared to those of ovan3amp·13tfib. Specifically, in the first solid, the I···N distance is of 3.014(5) Å with an angle C-I···N of 163.2(2)° whereas, in the second one, the same interaction is characterized by a distance of 2.861(4) Å and by an angle of 173.2(1)°. Distances for the I…O interactions follow the same trend (Table 1). We speculate that this aspect could account, in part, for the difference in the photochromic behavior of the two solids. As a matter of fact, these results suggest that when the N-salicylideneaminopyridine molecule is bound to both its moieties (pyridine and vanillidene), photochromism is unlikely to occur even for high values of Vfree. This aspect is corroborated by the fact that, although ovan3amp·12tfib is photochromic, the thermal fading of its coloration is much slower in comparison to the other two photochromic cocrystals ((ovan3amp)2·ofib and (ovan3amp)2·135tfib) reported in this study.

1:2 cocrystals with ditopic isomeric co-formers: (ovan3amp)2·14tfib, (ovan4amp)2·12tfib and (ovan4amp)2·14tfib. None of the cocrystals belonging to this group showed photochromic properties. Intermolecular

halogen

interactions

involve

the

pyridine

moiety

of

the

N-

salicylideneaminopyridine derivative. Vfree values range from about 87 Å3 for (ovan4amp)2·14tfib, to about 101 Å3 for (ovan3amp)2·14tfib. Two over three solids are non-photochromic and follow the Φ angle rule (near-flat N-salicylideneaminopyridines, see Table 4). Instead, solid (ovan3amp)2·14tfib is non-photochromic even if Φ > 30°.15 13 ACS Paragon Plus Environment


Interesting outcomes arise from

Page 20 of 31

the structural comparison of this latter with

(ovan3amp)2·ofib (see next section). Table 4. Molecular packing parameters and experimentally determined photochromism for (ovan3amp)2·14tfib, (ovan4amp)2·12tfib and (ovan4amp)2·14tfib. (ovan3amp)2·14tfib

(ovan4amp)2·12tfib (ovan4amp)2·14tfib

Φ/° 34.3(4)°

Vfree / Å3 101

ρ / (g cm-3) 1.850

photochromism No

10.9(5)° 2.0(4)° 6.3(5)°

106

1.828

No

88

1.918

No

Isomorphous cocrystals: (ovan3amp)2·14tfib and (ovan3amp)2·ofib Our recent work has been based on the rationalization of the effect of intermolecular interactions on photochromism.55 For this purpose, we preferred to work on isomorphous solids75 (and their solid solutions, when possible) in which we studied the effects of small “point” modifications on the solid-state photochromism.15 Rather surprisingly, we found that despite the isomorphism,15 N-salicylideneaniline solids can behave differently with respect to photochromism. For example, replacement of bromine with iodine led to loss of photochromism in isomorphous single-component solids. Replacement of bromine by iodine in cocrystals with p-diiodotetrafluorobenzene led, instead, to a slowdown of the thermal fading rate of the photoinduced coloration of about three times.15 This same approach has been used by other groups to study the thermal expansion76 of cocrystals, the rotational dynamics of molecular rotors77 and the thermomechanical effect in molecular crystals.78 Herein, we report an additional case of isomorphism which involves cocrystals (ovan3amp)2·14tfib and (ovan3amp)2·ofib (Figure 6 and Figure S5). Despite their isomorphism, only compound (ovan3amp)2·ofib is photochromic. Molecules of ovan3amp are stacked along the a-axis. These stacks of chromophores are then interconnected by bridging co-formers through halogen bonds directed toward the pyridine moieties. Stacking parameters of ovan3amp are similar in the two structures. Centroid distances (centroids calculated on the vanillidene ring) are of about 4.6 Å, for (ovan3amp)2·14tfib, and of about 4.8 Å, for (ovan3amp)2·ofib, with interplanar separation between molecules of about 3.4 Å and 3.5 Å, respectively. Both solids are characterized by a Φ > 30° (34.3(4)° and 37.3(3)° respectively) as well as by close values of Vfree (101 Å3 and 110 Å3) indicating that photochromism is hardly predictable on the basis of those parameters, given that small 14 ACS Paragon Plus Environment

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structural modifications can have a strong impact on the expression of the photochromic properties in the solid state. (Table 5).

Figure 6. Packing view along the a-axis of (a) (ovan3amp)2·14tfib (blue sticks) and (ovan3amp)2·12F8C4 (green sticks). The two solids are isomorphous.

Table 5. Molecular packing parameters and experimentally determined photochromism for (ovan3amp)2·14tfib and (ovan3amp)2·ofib. (ovan3amp)2·14tfib

Φ/° 34.3(4)°

Vfree / Å3 101

photochromism No

(ovan3amp)2·ofib

37.3(3)°

110

Yes

Cocrystals with the tritopic co-former: (ovan3amp)2·135tfib and ovan4amp·135tfib Cocrystallization of ovan3amp and ova4amp with I35tfib interestingly generated cocrystals with different stoichiometry. A 2:1 stoichiometry was found for the first cocrystal while a 1:1 was found for the second one. Solid (ovan3amp)2·135tfib is photochromic and the two crystallographically independent chromophores in its structure show Φ of 8.6(7)° and 38.2(6)°. This system fulfills the selection criterion of the Φ rule. Comparison of this solid with the 2:1 cocrystals discussed previously ((ovan3amp)2·14tfib, (ovan4amp)2·12tfib and (ovan4amp)2·14tfib) is of interest because the only difference in the molecular volume of the constituents is due to the substitution of a fluorine with an iodine atom. The Vfree value of (ovan3amp)2·135tfib is above the ones found for the other three 2:1 cocrystals (see Table 4 and Table 6), therefore its photochromic behavior is also in accordance with the Vfree model. Solid ovan4amp·135tfib is non-photochromic. The N-salicylideneaminopyridine molecules are planar with Φ = 7.2(4)°. For this reason, the cocrystal fulfills the Φ rule. When, instead, we compare this cocrystal to the two 1:1 cocrystals discussed previously (ovan3amp·12tfib and ovan3amp·13tfib), we notice that despite the fact that its Vfree has the largest value, the compound does not show photochromic properties. A closer look to the ovan4amp·135tfib structure reveals that molecules form dimers stacked in a heat-to-tail fashion with short interplanar distances (of about 3.4 Å). This structural feature has been linked with the



Page 22 of 31

absence of the solid-state photochromism in a class of N-salicylideneaminopyridinium derivatives. 69

Table 6. Molecular packing parameters and experimentally determined photochromism for (ovan3amp)2·135tfib and ovan4amp·135tfib. Φ/°

Vfree / Å3

photochromism


8.6(7)° 38.2(6)°

123

Yes

ovan4amp·135tfib

7.2(4)°

163

No

Conclusions A series of halogen bonded cocrystals were synthesized by a one-pot one-step mechanochemical procedure. Molecules were assembled following a 1:1 or a 2:1 stoichiometry and formed various types of supramolecular assemblies, namely infinite chains, rings (tetramer), and discrete three-membered assemblies. Of the eight cocrystals, three exhibited photochromic properties with retention of the photocoloration ranging from three hours (ovan3amp·135tfib) to several days (ovan3amp·12tfib). All the photochromic cocrystals remained photoactive during several photoinduced switching cycles. The

1:1

cocrystals

with

ditopic

isomeric

co-formers

(ovan3amp·12tfib

and

ovan3amp·13tfib) formed structures in which the N-salicylideneaminopyridine is bound to both its sides (pyridine and vanillidene) through halogen bonds. This assembly generated a non-photochromic cocrystal (ovan3amp·13tfib) and a photochromic one, this latter (ovan3amp·12tfib) characterized by an extremely slow thermal fading of the coloration. Its photochromic behavior could not be described by the sole evaluation of Φ rule (Φ = 13.0(6)°) nor by the evaluation of Vfree. Remarkably, ovan3amp·12tfib exhibited photochromism although it is denser (ρ = 2.110 g cm-3, Vfree = 135.9 Å3) than ovan3amp·13tfib (ρ = 2.010 g cm-3, Vfree = 160.8 Å3). This comparison suggests that when the N-salicylideneaminopyridine

molecule is strongly bound through both halogen acceptor sites (pyridine and vanillidene), photochromism is unlikely to occur, even for high values of Vfree. Alternatively, this assembly can give rise to photochromic systems with extremely slow rates of thermal fading of the photoinduced coloration.


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The

2:1

cocrystals

with

ditopic

isomeric

co-formers

((ovan3amp)2·14tfib,

ovan4amp)2·12tfib and (ovan4amp)2·14tfib) formed structures in which the Nsalicylideneaminopyridine derivative is involved in discrete 3-membered assembly through I…N interactions on the N-atom of the pyridine moiety. All cocrystals were found to be nonphotochromic, possibly because this stoichiometry led to compact packing with Vfree values ranging from about 87 Å3 to 101 Å3 (much below Vfree values found for the 1:1 cocrystals discussed above). Moreover, compound (ovan3amp)2·14tfib represented another exception to the Φ rule being non-photochromic despite Φ = 34.3(4)°. However, when 14tfib is replaced

by

ofib

in

the

structure

of

(ovan3amp)2·ofib

(isomorphous

to

(ovan3amp)2·14tfib)), the resulting cocrystal is photochromic. Although,

generally,

photoisomerization

of N-salicylideneaniline

derivatives

is

a

phenomenon depending on the compactness of the crystal packing – therefore explainable in some cases through Φ and Vfree – we have shown how subtle structural changes can dramatically affect photochromism in the solid-state. Some incongruences can also be attributed to the fact that Vfree does not provide any description of the localization of the voids into the unit cell. This last factor is of tremendous importance when working with multicomponent crystal forms because not all the free available space areas in a multicomponent crystal are actually/equally employed by the chromophore for the cis-trans isomerization. We do believe that the combined Φ/Vfree model, grossly oversimplifies the complexity of the

dynamic phenomenon behind the

photoisomerization of N-

salicylideneanilines. For the above-mentioned reasons, careful examination of intermolecular interactions and crystals packing (localization of voids, evaluation of intermolecular interactions and stacking modes) should be taken into account and used to supplement the current proposed model based on Φ and Vfree.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Analytical data (PXRD, UV−vis), crystallographic data (CIF) and ORTEP diagrams.

Acknowledgements



Page 24 of 31

Part of this work was carried out on XRD equipment from the PC2 platform of UNamur. Tom Leyssens (Université catholique de Louvain) and Benoît Champagne (UNamur) are gratefully acknowledged for fruitful discussion. A.C is grateful to Celine Smulders and Margaux Simon for assistance in the preliminary stages of this work. A.C is an F.R.S-FNRS (“Aspirant”) fellow. This work was published thanks to funding of Actions de Recherche Concertées (ARC) de la Direction générale de l'Enseignement non obligatoire et de la Recherche scientifique—Direction de la Recherche scientifique—Communauté française de Belgique and the Croatian Science Foundation under the project IP-2014-09-7367.

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For Table of Contents Use Only Playing with Isomerism: Cocrystallization of Isomeric NSalicylideneaminopyridines

with

Perfluorinated

Compounds as Halogen Bond Donors and Its Impact on Photochromism

Andrea Carletta1, Marija Zbačnik2, Mégane Van Gysel1, Matea Vitković2, Nikolay Tumanov,1 Vladimir Stilinović2, Johan Wouters*,1, and Dominik Cinčić*,2


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1

Department of Chemistry, Namur Institute of Structured Matter (NISM), University of

Namur, 61 rue de Bruxelles, B-5000 Namur, Belgium E-mail: [email protected] 2

Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, HR-

10000 Zagreb, Croatia. E-mail: [email protected]

TOC graphic

Synopsis:

Photochromism was studied in cocrystals of isomeric Nsalicylideneaminopyridines with perfluorinated halogen bond donors as co-formers. Our results suggest that the sole evaluation of Φ and Vfree model, grossly oversimplifies the complexity of the dynamic phenomenon behind the photoisomerization of Nsalicylideneanilines. Moreover, isostructural cocrystals of the same chromophore molecule may not show the same photochromic behavior.


Playing with Isomerism: Cocrystallization of Isomeric N

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